Traditional digital power meters typically employ conventional passive internal current transformers and resistive potential dividers in order to reduce relatively large input currents and voltages by a defined and calibrated ratio down to lower currents and voltages that are readily sampled and converted into a digital representation for further signal processing. Current transformers and resistive potential dividers additionally provide much needed electrical isolation between the external current and voltage signals being measured. With potential dividers, the isolation is afforded by providing a robust (transient overload) and high divider impedance (typically >1 Meg ohm) between the voltage source and the digital power meter input circuitry. Current transformation ratios of 1:1000 are common with (but not limited to) typical nominal primary current levels of 1, 5, or 20 Amps in the case of transformer-connected power meters. Voltage transformation ratios of 200:1 are common with (but not limited to) typical nominal voltage inputs ranging from 67 to 600 Vac. Accurate current and voltage transformation, in both magnitude and phase, is required, particularly when AC power calculations are being made at low power factors. Amplitude error of less than +/−100 ppm, combined with phase shift errors of less than +/−1 minute, are required by the newest generation of Class 0.1 digital power meters. Accuracies must be maintained over widely varying signal amplitudes and environmental conditions. Accuracy at higher current and voltage signal frequencies well beyond fundamental 60 Hz power signals are becoming common, particularly when harmonic representation, power quality, and transient analysis is required.
Conventional current ratio transformers suffer from a fundamental electro-magnetic limitation that directly impacts their effective use in modern sophisticated digital power meters, particularly the new class of power quality meters requiring high accuracy (Class 0.1), wide dynamic range, stability, and frequency response. This limitation is due to the fact that a portion of the primary input current being measured is required to magnetize the core. This magnetization current component is complex in magnitude and phase and directly impacts the ratio and phase error of the current ratio transformer output current. Core magetization effects may also impact accuracy by shifting the transformer flux swing operating point. Larger, high permeability cores are typically used in order to minimize the effects of core magnetization loss. These undesirable effects are only reduced and not eliminated through the use of such cores. Tape wound torroidal cores, made of ultra high permeability magnetic alloys, such as Molypermalloy, Supermalloy, and Amorphous Glass, may be required to meet the 60 Hz accuracy specifications, but issues of cost, size, and accuracy often limit their inclusion in new high performance designs.
A conventional potential divider used for power meter AC input voltage division typically utilizes high valued resistors in order to safely divide the input signal to low levels compatible with conventional electronic analog to digital conversion circuitry. The divider input resistor values must also be of high value in order to limit power dissipation under nominal and overload conditions while reducing leakage currents to safe levels. Unfortunately, the use of such high value resistor divider chains can result in temperature, humidity, capacitive, and thermal noise induced stability issues. The use of high precision matched resistive dividers (e.g., metal foil) are generally required for high accuracy applications but come at a high cost factor.
The continuing trend of increased digital power meter performance, particularly in areas of accuracy and frequency response, requires a new and improved approach.